RE: Kids Board with possible dual use??
The Starsurfer S and M are the exact same shapes as the original 2006 Starboard Aero 117 and 127 light wind wave and crossover boards. The heavier weight makes them less reactive and a bit more power hungry, but they are still very fin boards. If you will not use it as a beginner board for grown ups, I think the S version will be enough for the 11 year old and also be more fun for you and your wife (lots of sailing style option to explore on that board).